1. Field of the Invention
The present invention relates to a semiconductor memory device.
2. Description of the Related Art
In recent years, a semiconductor memory device, for instance, a NAND flash memory, has begun to be used as a memory for various electronic devices, such as portable audio devices, by virtue of its advantages of large capacity and nonvolatility (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2005-235260).
A self-boost method is known as a write method of the NAND flash memory. In this write method, a voltage of 0V or a power supply voltage Vcc is applied to a bit line. A drain-side select gate is set in an ON state at the power supply voltage Vcc, a source-side select gate is set in an OFF state at 0V, and a source line is set at an internal power supply voltage Vcc. Thereby, each of memory cell units is set at “0” (write state) or “1” (boost state).
On the other hand, in a read operation, a word line of a read cell is fixed at 0V. The other cells are set at a read voltage Vread, a drain-side select gate is set in the ON state at the power supply voltage Vcc, and a source-side select gate is cut off at 0V. Thereby, the bit line and wiring line are charged. Subsequently, the source-side select gate is set in the ON state at the power supply voltage Vcc. At this time, when the read cell is in the write state, the word line is at 0V, and the drain-side potential, which has been cut off and charged, does not vary. On the other hand, when the read cell is in an erase state, the word line cannot be cut off at 0V, and the word line is discharged to the source side, and the drain-side potential lowers. Data read is executed by monitoring the bit line potential by means of, e.g. a sense amplifier.
As has been described above, in the conventional structure, each bit line needs to be provided with one bit line contact. Since the pitch of neighboring contacts is determined by the bit line pitch, the processing of contacts becomes very difficult as the degree of microfabrication progresses. Thus, there is a tendency that the conventional structure is disadvantageous in terms of microfabrication.